Our I.ivlni^ Resources — Interior West 



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point for foraging because of the ease with 

 which spawning trout could be obtained by 

 eagles. 



The concentrations of wintering and migrant 

 hald eagles at Nankoweap are analogous to how- 

 eagles formerly concentrated at McDonald 

 Creek in Glacier National Park. Montana 

 (McClelland 1973). There, the introduction of 

 non-native kokanee salmon (Oiicorhynchus 

 ncrka) eventually attracted hundreds of migrant 

 bald eagles (McClelland et al. 1982). The^sub- 

 sequent introduction of exotic zooplankton into 

 Flathead Lake recently caused the collapse of 

 this salmon population and ended the concen- 

 tration of wintering eagles. In Grand Canyon 

 National Park, it was felt that if the number of 

 spawning trout remained high in the Colorado 

 River tributaries, bald eagles might continue to 

 concentrate there for food, as happened along 

 McDonald Creek at Glacier National Park. 



This article outlines the 1989-94 status of 

 wintering bald eagles along the Colorado River 

 corridor, from the Glen Canyon Dam through 

 Grand Canyon National Park. We also discuss 

 the trends of bald eagle numbers as determined 

 from monitoring eagle and fish populations 

 throughout the river conidor. 



We determined the annual status of bald 

 eagles from 1990 to 1994 by direct ground 

 observations from the river bottom at the con- 

 fluence of Nankoweap Creek and the Colorado 

 River, and from aerial censusing flights from 

 January through April. 



Trends 



Aerial Surveys 



Wintering bald eagles were present each 

 year along the Colorado River comdor from 

 late fall (October-November) through early 

 spring (March- April). During the 1990-91 aeri- 

 al censusing surveys, peak numbers occuned in 

 January and February, so aerial surveys in sub- 

 sequent years were confined to December 

 through March (Fig. 2). Eagles were observed 

 on every flight, with numbers ranging from 2 

 (in March 1993) to 23 (in February"l991 ). Bald 

 eagles were generally distributed evenly along 

 the river corridor except in January and 

 February, when conditions were suitable and 

 rainbow trout were spawning in tributaries 

 (Leibfried and Montgomery 1993). During 

 these 2 months birds concentrated at the small 

 tributaries. 



Ground Surveys at Nankoweap 



The trend of bald eagle numbers at 

 Nankoweap Creek was for birds to clo.sely par- 

 allel spawning trout numbers (Fig. 3). During 

 1990-91 we recorded the highest known bald 



eagle concentration at Nankoweap Creek with 

 up to 26 eagles present on a peak day (Fig. 4). 

 About 70-100 individuals were documented 

 during the eagle concentration (when at least 10 

 eagles were present each day) from 8 February 

 to 8 March 1 990. The previous high of 1 8 win- 

 tering eagles was recorded at Nankoweap in 

 February "l 988 (Brown et al. 1989). The trend 

 was for fewer numbers of trout and birds in fol- 

 lowing years (Fig 3). For example, in 1993 

 when spawning was extremely low in 

 Nankoweap Creek, there were concomitantly 

 low numbers of eagles. In 1994 spawning trout 

 numbers were also low in the creek and few 

 bald eagles were found in the area. 



Other Areas of the Plateau 



During 1992-94 when the numbers of win- 

 tering bald eagles along the Colorado River 

 were low (Fig. 3), concentrations of bald eagles 

 were reported at other locations on the southern 

 Colorado Plateau. For example, in 1992. 

 Bureau of Reclamation pilots (personal com- 

 munication) noted eagle concentrations at the 

 junction of the Green and Colorado rivers (Fig. 

 I). During 1993 the Arizona Game and Fish 

 (personal communication) reported up to 20 

 eagles at Lake Mary, just east of Flagstaff, 

 Arizona. These birds were feeding on some of 

 the thousands of rainbow trout the agency had 

 stocked into the lake during the winter. In 1994, 

 another year of low bald eagle numbers along 

 the Colorado River corridor, we received 

 numerous reports from state and federal agency 

 biologists of small eagle concentrations at elk 

 and deer carcasses over the southern Colorado 

 Plateau. 



Status 



The status of bald eagles along the Colorado 

 River, especially in portions of Grand Canyon 

 National Park and Glen Canyon National 

 Recreation Area, has been improved by an 

 increase in numbers of introduced rainbow trout. 

 For example, at Nankoweap Creek, the trend 

 went upward from a few birds starting in the 

 mid-1980's to peak numbers in 1 990-9 f. In fol- 

 lowing years (1992-94), poor rainbow trout 

 spawning resulted in low numbers of bald eagles 

 in this region. Creek morphology and flow con- 

 ditions varied among years and influenced the 

 availability of trout, and thus eagle numbers. 



Bald eagles at Nankoweap. however, can be 

 the largest such concentration in the southwest- 

 em United States. The 70-100 individual eagles 

 recorded during 1990 represent what is believed 

 to be one-fourth of the entire population of bald 

 eagles wintering to the south of the Grand 

 Canyon (in Arizona and northern Mexico). We 

 expect that wintering eagles will continue to 



25- 



S. 10 



Dec Jan Feb Mar 

 Months 



Fig. 2. Average number of bald 

 eagles detected each month ( 1990- 

 94) during aerial surveys along the 

 Colorado River from the junction 

 of the Little Colorado River north 

 to Glen Canvon Dam. 



90 91 92 93 

 Year 



Fig. 3. Average daily numbers of 

 rainbow trout and bald eagles at 

 Nankoweap Creek in Grand 

 Canyon National P;irk. AZ. 1990- 

 94. 



— 1990 



1991 —1993 



10 20 ' 10 20 

 January February March 



Fig. 4. Bald eagle abundance at 

 Nankoweap in 1990. 91. and 93. 



